Ammonia is the second largest chemical synthetic product with approximately 400 ammonia plants operating globally. H2 and N2 are reacted at a temperature of between 400 and 500° C. and a pressure greater than 100 bar over an iron based catalyst The production of the nitrogen and the hydrogen is the single most expensive step in the synthesis process. Much effort has hence been devoted to reducing the cost of synthesis gas production. Liquified Petroleum Gas (LPG), naphtha, petroleum coke, coal and natural gas have been used as feedstocks, although the vast majority of processes utilise natural gas as feedstock and fuel. CO, CO2 and H2O are all considered poisons for the catalyst even at parts per million concentration levels and hence great effort is made to remove them from the synthesis gas. Sulphur, particularly in the form of H2S also acts as a permanent poison and has to be removed from the synthesis gas to very low levels.
State of the art ammonia synthesis processes use a two step reforming process. The primary reformer is an indirectly heated tubular reactor filled with a Ni catalyst The natural gas is passed through this reactor after being mixed with steam. The reaction is controlled so that there is about 15% methane remaining in the exit stream. The partly reformed gas is then transferred to the secondary reactor. Air is added in a burner, and the oxygen and methane react exothermically. The hot gas passes adiabatically through a catalyst bed and exits at approximately 1 000° C. The resultant synthesis gas contains CO, CO2, H2, N2, H2O and small quantities of CH4 and other unconverted hydrocarbons.
The synthesis gas is cooled and passed through first a high temperature shift converter operated at 320-350° C., then further cooled and passed through a low temperature shift converter. The combination converts almost all of the CO into CO2 and H2 by reaction with water. The gas is then treated to remove CO2 using a suitable solvent A number of commercially available technologies can be utilised for this step. The solvent is regenerated by flashing, and the CO2 is vented. Methanation is used as the final treatment step. In this unit, almost all of the remaining carbon oxides are reacted with H2 to form methane and water. The water is removed using molecular sieve absorbers.
Some ammonia synthesis technology vendors (eg. Linde and KTI) do not use a secondary reformer as described above and use pressure swing adsorption (PSA) to separate H2 from the other synthesis gas constituents. The N2 is supplied from a cryogenic air separation unit. Overall efficiencies for this process are reported to be as good as conventional technology.
Synthesis gas comprising predominantly CO and H2 can be used for the manufacture of liquid hydrocarbons utilising Fischer-Tropsch Synthesis. Hydrocarbons are typically produced by contacting synthesis gas with a preferred selective catalyst such as Co or Fe at 200-260° C. and 10-50 bar. Although it is known that the Fischer-Tropsch reaction can be performed in the presence of N2, it is in general not preferred. In the process, N2 acts as an inert gas that lowers the reactant partial pressures, and thus larger reactors or more catalyst is required. The selectivity to heavier hydrocarbons is also negatively affected by large concentrations of inert gases. As with ammonia synthesis, the synthesis gas for hydrocarbon production is typically produced from a natural gas feedstock utilising steam methane (tubular) reforming, autothermal reforming, or a combination of the two.
The process in which synthesis gas is produced for hydrocarbon production utilises a high purity O2 stream in the reforming step rather than air, because the addition of inert gases (particularly N2) is generally considered detrimental to the process economics. In stand alone Gas-to-Liquids (GTL) plants, nitrogen is an unused byproduct of the air separation step. GTL plants are very intensive oxygen users, with between 0.2 and 0.3 tonnes of oxygen consumed per barrel of product. Consequently between 0.5 and 0.7 tonnes of nitrogen per barrel of hydrocarbons is made available.
As with the ammonia synthesis catalyst, the Fischer-Tropsch catalyst is highly sensitive to poisoning by sulphur compounds and these have to be removed so that only extremely low levels remain to ensure economic catalyst life.
It is known in the art that under most conditions, the Fischer-Tropsch process requires a synthesis gas that contains H2 and CO in a ratio at, or below, about 2.5, and more preferably at or below 2.0. This is because when certain catalysts are used, for example a Co catalyst, better selectivity for heavy hydrocarbons is achieved when the above ratio is at or below 2.0. Conventional steam reforming and autothermal reforming technologies produce synthesis gas at a ratio greater than this ideal. Various alternatives have been proposed to obtain the correct ratio. These involve recycling of CO2 which can be extracted from various points in the synthesis loop or recycling the Fischer-Tropsch tailgas back to the reforming section. Such methods are useful not only because they reduce the H2/CO ratio, but also because they increase the overall carbon utilisation in the process.
Conversely, ammonia synthesis requires a very high H2/CO ratio. This is adjusted even further after the reforming section by using shift converters, which convert CO and water into CO2 and H2. The CO2 is extracted using known methods and is vented to the atmosphere.
The ammonia synthesis process has been used in combination with a Fischer-Tropsch process commercially only in one case (by Sasol). In this process, the tail gas exiting the Fischer-Tropsch reactor is used as the source of hydrogen. After treatment of this gas to remove hydrocarbons, a portion is sent to a shift converter. H2 is recovered and this is then reacted with N2, obtained from a cryogenic oxygen plant, in the ammonia synthesis process. This process is useful when:    1) the synthesis gas enters the Fischer-Tropsch reactor with a H2/CO ratio greater than the stoichiometric ratio. The stoichiometric ratio is the ratio of H2 used to CO used in the Fischer Tropsch reactor. When the synthesis gas enters the reactor with a high ratio, H2 builds up and the tail gas contains a higher proportion of H2 than the feedgas.            (A number of reactions influence the ultimate stoichiometric ratio, for example, the production of various hydrocarbons and the water gas shift reaction:(2n+1)H2+nCO→CnH2n+2+nH2O  (paraffin production)2nH2+nCO→CH3[(CH2)n−3]CH═CH2+nH2O  (olefin production)2nH2+nCO→CnH2n+2O+(n−1)H2O  (alcohol production)CO+H2O→CO2+H2  (water gas shift))        
Each specific catalyst and the particular process conditions determine the ultimate stoichiometry of H2 and CO utilisation because the relative rates of each of the reactions varies. In general, however, it is well known in the art that Fe based catalysts are active for the water gas shift reaction, while Co based catalysts are. not Thus the stoichiometric ratio for Co catalyst is close to 2.0, whereas it is somewhat lower for Fe based catalysts;    2) the conversion is low so that not all of the available H2 is utilised and can thus be extracted from the tail gas for ammonia synthesis.
Modern GTL facilities are designed to primarily produce liquid fuel. This is achieved in a three step process involving a) synthesis gas generation, b) hydrocarbon synthesis and c) hydroprocessing. The processes are designed to be highly efficient with high conversions and good selectivity so that the liquid fuel product can compete economically with conventional fuels derived from crude oil. Thus in a modem GTL facility, the above Sasol process will not be suitable as options (1) and/or (2) above are not considered to be viable.
There therefore remains a need for optimising the conversion of natural gas to synthesis gas so that desired H2/CO ratios are obtained for use in both the production of hydrocarbons and the production of ammonia.